[DEV-2153] Refashion "People" page into a leaderboard.

@mwhansen cr?

The leaderboard template is mostly a copy-paste of the user list page template from Askbot, with some pieces removed and a few pieces that were previously in macros inlined and modified.
This will only be correct if ```request.user``` appears on the current page due to the current implementation of ```get_leaderboard_position```.